Re: gvalues

[Owen Taylor <otaylor redhat com>]

On Thu, 2003-04-24 at 12:34, suk wrote:
> I'm writing about minor performance disadvantage when accesing
> style_property values. So it goes:
> 
> This code pattern is at several places. Example from GtkButon.c
> 
>  static const GtkBorder default_default_outside_border = { 0, 0, 0, 0 };
>  static const GtkBorder default_default_border = { 1, 1, 1, 1 };
> 
>  .....
> 
> static void gtk_button_get_props (GtkButton *button,
>                       GtkBorder *default_border,
>                       GtkBorder *default_outside_border,
>                       gboolean  *interior_focus)
> {
>   GtkWidget *widget =  GTK_WIDGET (button);
>   GtkBorder *tmp_border;                       
>   if (default_border)
>     {
>       gtk_widget_style_get (widget, "default_border", &tmp_border,NULL);
> 
>       if (tmp_border)
>         {
>           *default_border = *tmp_border;
>           g_free (tmp_border);
>         }
>       else
>         *default_border = default_default_border;
>     }
> ....
> 
> This function is called from other code like this:
> 
>   GtkBorder default_border;
>   gtk_button_get_props (button, &default_border, NULL, NULL);
> 
> 
> This tries to get_style_property and if not found then return some
> fallback value. But it unreasonably allocates/frees memory and copies
> the value twice (branch when property exists) and once otherwise.
> This of course goes against the idea in gtk_style_... to have access
> to values fast.
>
> Variant 1:
> 
> So maybe it would be better to have
>   boolean  gtk_widget_style_get_property (widget, "default_border", &variable );
> instead of
>   void gtk_widget_style_get_property (widget, "default_border", &variable );
> 
> 
> With this we can make it with only one copying (in each branch one).
>       if (!gtk_widget_style_get (widget, "default_border", default_border, NULL))
>             *default_border = default_default_border;
> So one copying is done inside gtk_widget_style_get 
>         - but this time, without alocating new struct.
> and if there is no such property, we do copying outside.

Perhaps you are confusing GtkBorder ** and GtkBorder *. There is no
handling of structures _by value_ in GObject, you have to always
deal with pointers to structures. ("Boxed types")

You can think of that call to gtk_widget_get_style() like it
had a signature such as:

 gtk_widget_style_get_property (GtkWidget *widget, 
                                const char *property_name
                                GtkBorder **default_border);

> I don't understand G_VALUES properly, but this would be fairly easy for
> someone who does. 
> 
> Variant 2:
> Of course we can make no copying at all. Because 
> 
>         void* gtk_widget_style_get_property (widget, "default_border" );
> 
> gets the value from style->property_cache, so if we promise to not touch
> and not store for a long time.... This variant will not get through, i'm
> sure.

The reason that we have to return a newly allocated GtkBorder structure
rather than a pointer to something internal is that we always have the
same allocation policy every time we we go through the property
system. You can't say "not making a copy is OK in this case, but in
this case, we need to make a copy you can free".

I don't think this stuff is worth worrying about unless it shows
up on profiles, honestly.

Regards,
                                                Owen